In conjunction with the small-scale, melt/concrete interaction tests being conducted at Sandia Laboratories, an acoustic technique has been used to monitor the penetration of a molten, metallic pool into concrete. Real-time plots of the position of the melt/concrete interface have been obtained, and they illustrate that the initial penetration rate of the melt may be on the order of 80 mm/min. Phenomena deduced by the technique include a nonwetted melt/concrete interface.